\chapter{Conclusion}

The literature on the subject of irreversibility of combustion gives no unified answer as to which mechanism in turbulent diffusion flames (or other flame configurations for that matter) gives the highest contribution to the total entropy production. For premixed flames many authors suggest that irreversibilities from heat diffusion by conduction is the largest contribution to entropy production \cite{Dunbar1994}. In general the entropy production from each individual mechanism is higher for turbulent combustion than for laminar combustion, and higher for nonpremixed combustion than for premixed combustion \cite{Som2007}.

Simulations where performed on a 2D grid of 72 cells. The aggregated results of the simulation using the fast chemistry assumption are given in Table \ref{tab:fastTotalEntropy}. The percentage contribution of the total mean entropy production rate for heat transfer is 38.5 \%, for chemical reaction 37.4 \% and for mass transfer 24.1 \%. 

The simulation was also run with curve fits of the density averaged temperature and mass fractions from experimental measurements. The result from this simulation is given in Table \ref{tab:curvefitTotalEntropy}. The percentage contribution of the total mean entropy production rate was for heat transfer 6.5\%, for chemical reaction 85.9\% and for mass transfer 7.6 \%. The total irreversibility rate for this approach was almost 20 times higher than when fast chemistry was assumed.  The assumptions made for calculations affect the results significantly. The exergy efficiency is found to be 76.0 \% for the curvefit approximation and 98.7 \% when fast chemistry is assumed. 

In general the rate of entropy production is controlled by the temperature and concentration gradients of the flame. Increasing the temperature of the coflow reduces the exergy destruction due to heat diffusion. Decreasing the concentration gradients will give a reduction in the entropy production from mass diffusion, Eq. \ref{eq:massentropy}. 

Chemical reaction in a diffusion flame happens in a thin layer where the mixture of fuel and oxidizer is close to stoichiometric. The resolution of the simulations was limited by the resolution of the experimental data, and therefore the contribution of chemical reaction to the total entropy production might have been overestimated.

The prescribed PDF model seems to work for the flame in question, but if increased accuracy is demanded the fact that differential diffusion takes place in turbulent hydrogen combustion and that $Le<1$ should be accounted for. The second law efficiency when fast chemistry is assumed is a lot higher than what a bulk analysis suggests it should be.

Combustion processes taking place over a larger volume with no steep temperature or concentration gradients are well suited for efficient combustion. Most of the entropy production in a diffusion flame happens in the lower part of the flame in the thin reaction layer. Increasing the stoichiometric flame temperature reduces entropy production from chemical reaction and is also advantegeous from a bulk exergy point of view. 